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Whether communication is “instant” doesn’t matter because it takes the speed of light to observe.

Say A and B are entangled, both observers of A and B know they are entangled, A and B don’t have a value yet, there’s only A != B.

When A gets measured, it takes the speed of light for A’s measurement to reach B. If B gets measured before then it’s still a 50/50 (and then when A is revealed it will 100% be the opposite of B).

It’s true that if B gets measured B’s observer will instantly know A. But this doesn’t break the speed of light because B’s observer already knew that A != B, so they don’t need any more information from A to determine it when they measure B. Like how, if B and A were predetermined (not quantum) and then separated and B was revealed, B’s observer will instantly know A.

It rules out local hidden variables, non local hidden variables (eg Bohmian mechanics) is still allowed

I think the point being made is a non-local variable is just "instant communication" in a different guise. It's a variable that everyone can see, and once it's value is observed is instantly for all observers no matter how far apart.

Well the idea of hidden variables is that they are totally inaccessible to current technology, so they aren't really observable. In the case of Bohmian mechanics the "hidden variable" is the configuration of all particles in the universe, which is clearly not something that can be observed by any observer. It's also not a quantity that is changed by observation which your line "once it's value is observed is instantly for all observers no matter how far apart" implies. It isn't a quantum observable: it is a normal classical kind of state. It's just that the physics for one particle depends on the configuration of all other particles, which is completely non-local.

To me all these references to Bohmian mechanics, hidden variables and faster than light communication makes hides more than it reveals.

These connections always arise because of some invariant our universe preserves, eg charge is conserved, or spin is conserved, or whatever.

The universe happens to let us create particles - an electron say, from pure energy. But the conservation of charge means you can't just create an electron, there has to be a positron too otherwise charge isn't conserved. They would instantly annihilate each other if they hung around together, so in order for us to see them they must be moving away from each other (with equal but opposite momentum, because that's conserved too).

Until we measure it, we don't know what the charge of either of particles is. Quantum superposition means that until we measure it, the charge of both particles is effectively in a 3rd state: unknown (as opposed to positive or negative).

Putting this in terms of hidden variables, there is a variable that is in one of three states: "particle-A: Unknown charge, particle-B: Unknown charge", "A: positive, B: negative", "A: negative, B: positive". It doesn't look to me like this this variable exists everywhere. It only describes the state of these two particles which by definition occupy a tiny positions in space-time. And no it isn't unobservable either: you can observe the variable by just measuring one of the particles.

It does raise all sorts of interesting questions about how the universe operates. How does it preserve these invariants across space and why are they preserved? Does this quantum superposition look anything like it's being described here? I've seen other descriptions that make it arise quite naturally from the math of 2-norm probabilities and thus not require multi-verses or some mysterious "collapse" (which look to me to be about a useful as explaining what s going on as "Bohmian mechanics").

So these hidden variables are a hand wavy way of describing how conversation laws interact QM superposition. It's not really that complex - is it? Was there any need to introduce "hidden variables" at all?

It seems to me a lot of the difficulty people have in describing QM is not in QM itself, but from the knots people get themselves into in their personal struggle to understand it. They write their knots down, we gumbies read it and say "bugger me that looks hard".

QM nonlocality isn't useful for instant communication though.

If it is a physical process, you still cant influence it on one end in order to communicate faster than the speed of light.

And nothing is "observed instantly" since that phrase doesn't have meaning in SR.

Two space-like separated observations, which cannot communicate with each other in SR, will have correlated measurements. Which measurement comes first or if they happen at the same time, will depend on your reference frame and motion.

So once a single measurement has been made somewhere in the universe, all observers (light-like or space-like separated) will measure entangled values that agree with that measurement, but the entangled state was created indeterminately. If the observations are space-like separated you cannot say which one "caused" the collapse of the wave function.

And like I said you can't use it for communication. The fact that you can't assign which observation was cause and which one was effect is probably tightly tied to the fact that you can't use it to communicate -- which side is the sender and which side is the receiver? That depends on the reference frame, which produces nonsense, so to avoid a paradox it is banned.

Doesn't it rule out local non-superdeterministic hidden variable theories?

Yes

John Stewart Bell in 1964 proved that broad classes of local hidden-variable theories cannot reproduce the correlations between measurement outcomes that quantum mechanics predicts. The most notable exception is superdeterminism. Superdeterministic hidden-variable theories can be local and yet be compatible with observations.

There are quite a few exceptions that Bell's theorem doesn't cover. In addition to superdeterminism, it doesn't cover the case where the measurement equipment becomes entangled with the system being observed (in other ways than making a measurement). In addition, if the universe is not strictly quantized and the appearance of quantized measurements comes about through certain "preferred" resonant modes determined by the structure of the universe, we would see the measurements predicted by quantum mechanics despite having only purely local state (seemingly violating Bell's theorem, but since Bell's theorem assumes a quantum universe it doesn't actually apply)

No it doesn't.

It's perfectly explained by you becoming entangled with the system when you interact with it. No spooky action at a distance needed.

The wavefunction exists. Entanglement exists. Why is it so hard to extend that concept to the experimenter?

That’s superdeterminism (the version of it Sabine likes), but most scientists won’t accept it because they hear you as saying they can’t do reproducible experiments.

No it's not, it's many worlds. It's completely local and says nothing about free will or causality.

Step 1) Entangle pair. Parts of wavefunction with up-down and down-up exist.

Step 2) Lab A interacts with pair, they either get entangled with the up-down pair or the down-up pair, or any other subset of the wavefunction.

Step 3) Lab B interacts with any part of the Lab A+pair wavefunction. When they do, they find that, astonishingly, the part of the wavefunction they find themselves entangled with when they speak to Lab A is the same part of the wave function they find themselves entangled with based on their measurements.

No new entities are posited, no new mechanism is posited, no assertions are made about wavefunctions vanishing upon interaction. It's the simplest possible claim. The only effect it has on whether or not you can do reproducible experiments is the entropy in the subset of the wavefunction you can potentially interact with went up.

Or non-locality. In some sense things in the world aren’t as far apart as they seem.